Modern portable radios commonly feature compatibility with several accessories which may be connected to the radio, such as external microphones, speakers, or even personal computers. Such compatibility requires some means for connecting the internal circuitry of the radio to the circuitry of the external accessory. In addition, when an accessory is connected to the radio it becomes necessary to inform the control means of the radio of the accessory that has been connected, so that the operation of the radio can be adjusted accordingly.
Apparatus for selecting modes of operation for a radio depending on the external device connected to the radio are known. For instance, U.S. Pat. No. 4,792,986, to Garner, describes an option selection scheme using analog voltages, with an impedance device determining the voltage for an option. Each option would in turn use a different impedance, and thus generate a different voltage. There are, however, some serious limitations to this scheme. While the above scheme theoretically could have an infinite number of discrete voltages, practical limitations severely limit it. With the resolution available with low cost Analog-to-Digital convertors, and the noise present in the radio environment, only about 4 discrete levels are usable with voltage "windows." Further, conductive contamination of the option connector can cause false readings and errors. Thus, a need exists for apparatus for identifying the external accessory for the radio.